UBIFS: use ro_mount instead of MS_RDONLY

We have our own flags indicating R/O mode, and c->ro_mode is equivalent
to MS_RDONLY. Let's be consistent and use UBIFS flags everywhere.
This patch is just a minor cleanup.

Additionally, add a comment that we are surprised with VFS behavior -
as a reminder to look at this some day.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>